The Variable Spectrum of the Yellow Hypergiant rho Cassiopeiae

We have analyzed high-resolution optical spectra of the cool hypergiant rho Cas covering a period 2 years. The aim of this analysis is to check the range of the effective temperature variations with the pulsation of the atmosphere, in order to study variable emission components in Hα, to investigate the splitting of metallic absorption lines and to compute abundances of Na, Fe and other elements. We found an upper range of Delta T_eff ~= 750 K over a period 17 months, whereas the effective temperature change within a single pulsation period remained limited to 400 K. We discuss the notorious splitting of low excitation metallic absorption lines observed for rho Cas. The allowed emission reversals in the cores of these low energy lines emerge from cool and static shells in a bipolar stellar wind. Furthermore, variable absorption in the supersonic stellar wind recurrently produces far violet extended line wings with the atmospherical pulsations, from which we have derived the mass-loss rate and wind extension above the photosphere. The emission components of Hα suggest the presence of a thermally excited outer atmospherical region (a variable quasi-chromosphere). Several observed parameters (like the mass-loss rate) can be derived theoretically if we assume that the observed ``microturbulent'' line broadening is not caused by stochastic small-scale turbulent motions (the classical notion of microturbulence) but by thermal motions in stochastically distributed high-temperature sheets behind the many shocks.